1. Field of the Invention
The present invention relates to a valve and in particular to a valve of the type which is connectable to common flow path for inspiration and expiration gas in a patient ventilation system to provide separate the flows paths of the inspiration and expiration gases.
2. Description of the Prior Art
Known patient ventilation systems, such as respirators, ventilators and anaesthetic delivery systems, have associated tubing circuits. These tubing circuits commonly include an inspiration gas line for delivering breathing gas from the ventilation system to a patient and an expiration gas line for taking expired gas from the patient to the air or other expiration gas receiving facility. In common usage the tubing circuit may need to be frequently removed for cleaning or replacement, perhaps on a daily basis, in order to minimize the possibility of bacterial growth within the circuit which could infect the patient. Additionally, long lengths of flexible tubing in an inspiration branch and a,separate expiration branch make the determination of the patient""s tidal volume and lung-thorax compliance difficult since the tubing tends to expand and contract radially in response to pressure variations during a breathing cycle. Patient comfort needs also to be considered and some patients, especially xe2x80x9chome-carexe2x80x9d patients, wish to move around in their xe2x80x9ccare environmentxe2x80x9d, and obviously the smaller the amount of tubing the easier and more comfortable it will be for them as they move. It is therefore desirable to minimize the length and the number of tubes within the circuit.
This may be achieved by providing a common flow path for the inspiration and the expiration gases which therefore replaces two gas lines with only one for a part of the tubing circuit. In order to be able to do this, a valve arrangement is required that is able to separate inspiration and expiration gas flows in order to prevent the patient from re-breathing expired gases, or to enable the inspiration gas and the expiration gas to be treated separately within the ventilation system.
One known valve which is used in ventilation systems to achieve this result is disclosed in U.S. Pat. No. 5,002,050. This valve has a valve body with an inlet for inspiration gas, an outlet for expiration gas and a common inlet/outlet for connecting the valve to the airway of a patient. Concentrically arranged inner and outer gas flow conduits are also provided, each having one end connected to the common inlet/outlet via a common gas flow passage and opposite ends respectively connected to the inlet and the outlet. A differential area valving means is located towards the end of the inner conduit and is arranged so that inspiration gas always impinges on its larger surface area. This inspiration gas provides operational control of the valve so that its opening and closing is dependent on the pressure exerted by the inspiration gas on the valving means. The valving means is slidable along the common axis of the valve in response to the flow of pressurized inspiration gas through the valve to seal the end of the common gas flow passage and block gas communication between the valve outlet and the common passage. When the pressure exerted on the differential area valving means by the expiration gas exceeds that exerted by the inspiration gas, the valving means slides in the opposite direction to unblock the end of the common passage way and permit egress of expiration gas past the valving means, into the outer conduit and out of the valve through the outlet. Further check valves are attached to the differential area valving means which permit only a unidirectional flow through the valving means.
This arrangement, however, is mechanically complex and the seal itself is made over a relatively small area, which is the end of a pipe, so that small particles may prevent a proper seal being formed. Moreover, once fabricated, and the differential area of the valving means set, the operation of the valve is controllable mainly by varying the relative pressures of inspiration and expiration gas acting on the different areas. Thus, the operation of the valve is controlled basically by varying the amount of gas within the inspiration line of the tubing circuit. This line has a relatively large volume, compared with that of the valve, and this leads to a relatively slow response of the valve to inspiration gas changes.
A further known valve is disclosed in U.S. Pat. No. 5,538,002 which has a pair of concentric tubes arranged to form a narrow channel therebetween through which breathing gas is supplied to a patient. The inner tube is arranged to connect the respiratory system of the patient to the outside and an inflatable cuff is disposed within the inner channel to expand and seal against itself as breathing gas passes through the outer, narrow channel. A disadvantage with this self-sealing arrangement is that a relatively poor gas seal may be formed. This is because if the cuff is substantially fully expanded to form the seal then the region of contact between the two surfaces is small, and if the cuff is less expanded wrinkles may form in the contacting surfaces which can lead to a poor seal or to the surfaces being separable at low pressures.
It is an object of the present invention to provide a structurally simple valve, useable in patient ventilation systems, in which a relatively large sealing area can be provided and in which operation of the actual valving components occurs substantially independently of the relative pressures of the inspiration and expiration gases within the tubing circuit.
The above object is achieved in accordance with the principles of the present invention in a valve having a substantially parallel, concentric arrangement of an inner fluid flow passage and an outer fluid flow passage for respectively conducting fluid through the valve to and from a common flow conduit, in opposite directions in the respective passages, and a cuff which is inflatable to control the flow of fluid through the valve, the cuff being inflatable to form a fluid-tight seal against one or the other of the facing surfaces of the inner and outer flow passages, so as to block fluid flow through the outer passage.
Thus, by providing a valving element in the form of an inflatable cuff that seals between overlapping regions of the outer surface of an inner conduit and the inner surface of an outer conduit which are arranged to be substantially parallel to, and preferably co-axial with one another, a seal is formed across a large area of the external surface of the cuff. Additionally, by having the inside of the cuff connectable with an external pressurized fluid source then the opening and closing of the valve need not rely on either of the inspiration gas or the expiration gas. Moreover, the valve is of a relatively simple construction with the valving element having only one moving part i.e. the inflatable cuff.
Preferably the degree of inflation of the cuff is controllable to provide a variable area flow restriction which may serve, for example, as a Positive End Expiratory Pressure (PEEP) valve useable in a ventilation system to regulate the exhaled gas flow to keep the lungs at a positive pressure throughout the expiratory phase of a patient""s breathing cycle. Such PEEP ventilator operating modes are well known in the art but are achieved here in a novel manner. Additionally or alternatively, the variable area flow restriction may be used in conjunction with a differential pressure flow meter so that the size of the inflated cuff is varied in order to maintain the linearity of the meter over a large dynamic flow range. The operating principles of variable area flow restriction flow meters are well known in the art and are described, for example in U.S. Pat. Nos. 4,938,077 and 4,006,634.
The cuff may be formed integrally with the valve, attached to one or other of the inner and the outer conduit so as to provide a unitary design. This may be achieved, for example, by locating the cuff on the outer surface of the inner passage so that it inflates to seal against the inner surface of the outer passage.